R Code: Apply a function to a vector in an accumulative manner

cum.func <- function(x, func)
{ 
    ## Purpose: Apply a function to a vector in an accumulative manner (like cumsum)
    ## Arguments:
    ##   x: a vector
    ##   func: a function 
    ## Return: a vector that is the result of:
    ##   c( func(x[1]),  func(x[1:2]),  func(x[1:3]),  func(x[1:4]),  func(x[1:5]), ... )
    ## Author: Feiming Chen, Date: 30 Jan 2017, 14:06
    ## ________________________________________________
    
    N <- length(x)
    y <- rep_len(NA, N)
    for (i in 1:N) y[i] <- func(x[1:i])
    y
}
if (F) {                                # Unit Test
    identical(cum.func(5:1, mean),  c(5.0, 4.5, 4.0, 3.5, 3.0))
}